This invention relates to a record carrier upon which optically reproducible information is recorded and, more particularly, to such a carrier wherein information is represented by spaced apart pits which are recorded in substantially concentric circular tracks on the carrier, and to a method and apparatus for producing same.
Recently, various techniques have been proposed for recording information on a record carrier with a relatively high recording density and for reproducing such information therefrom. As an example, video information is recorded in spiral or substantially concentric circular tracks on a rotatable record disc. In one such proposal, the recorded video information is optically reproduced from a rotating record disc. This is achieved by recording the information in the form of optically detectable pits in the substantially circular tracks on the disc. In a reproducing operation, successive tracks are scanned by a light beam, and the intensity of the beam which either is reflected from the surface of the disc or is transmitted through the disc is modulated by the recorded pits.
A high recording density is preferred, especially when video information is recorded, in order to provide a record disc of relatively long playing time. As an example, if one frame of video information is recorded in each track, then, for the NTSC system, thirty frames per second must be reproduced, thereby requiring an angular velocity of the disc of 1800 rpm. This means that 1800 tracks are scanned each minute, thereby requiring an extremely small track width to permit a sufficient number of frames to be recorded so that a reasonable playing time for the disc is established. Since each pit represents a quantum of video information in a frame, the length and spacing of the pits must be carefully selected to enable a frame of information to be recorded in each track.
In one technique for recording video information in the form of pits in concentric tracks, the video signal is frequency modulated and the pits represent such frequency modulations. More particularly, in this prior art technique, a clipping level is established and the frequency modulated video signal is compared to this clipping level. The duration of the frequency modulated video signal which exceeds the clipping level is recorded in the form of a pit such that the length of this pit is proportional to that portion of the frequency modulated video signal which exceeds the clipping level. As the frequency component of the frequency modulated signal increases, the length of the pit decreases. Conversely, as the frequency component of the frequency modulated video signal decreases, the pit length increases.
In addition to varying in accordance with the frequency component of the frequency modulated signal, the length of a pit also is dependent upon the radius of the track in which it is recorded. This is appreciated by assuming that the same frequency component is recorded in a track whose radius is, for example, 15 cm. and a track whose radius is, for example, 5 cm. In both cases, the duration of the frequency modulated video signal which exceeds the clipping level is identical. However, the linear speed of the track whose radius is 15 cm. is three times greater than the linear speed of the track whose radius is 5 cm. Since the length of a recorded pit is proportional to the duration of that portion of the frequency modulated video signal which exceeds the clipping level multiplied by the linear velocity of the track, the length of the pit recorded in the track whose radius is 15 cm. is three times as long as the pit which is recorded in the track whose radius is 5 cm.
In view of this dependence of the pit length upon the radius of the track in which it is recorded, errors may be introduced during signal reproduction because the relationship between the length of a pit and the size of the scanning beam spot varies. For example, let it be assumed that the size of the scanning beam spot remains constant for the scanning of all tracks regardless of the radius of the particular track which is being scanned. Let it be further assumed that the recorded pits are detected by sensing the intensity of light which is reflected from the surface of the disc as the beam scans the track. Then, in the absence of a pit, that is, in the space between adjacent pits, the intensity of the reflected light is of one level and the intensity of the light which is reflected when the beam spot scans a pit is of another level. A pit is effectively detected when a portion of the scanning beam spot impinges thereon. Hence, if the length of a pit recorded in a track whose radius is, for example, 15 cm. is represented L.sub.1, and if the diameter of the scanning beam spot is represented as r, then the pit will be detected when the beam first impinges upon the leading edge of the pit and this detected condition will be maintained while the beam overlaps the pit and until the beam no longer impinges upon the trailing edge of the pit. Hence, the effective detected length of the pit is equal to L.sub.1 +2r. This also holds true when the beam of constant diameter r scans the pit of length L.sub.2 which is recorded in the track whose radius is 5 cm. Hence, in this latter case, the effective detected length of the pit is equal to L.sub.2 +2r. If the pits in these respective tracks represent the identical frequency component, then the length of each pit insofar as its length is determined by the frequency component is equal. However, since the pit of length L.sub.1 is recorded in a track whose radius is three times the radius of the track in which the pit of length L.sub.2 is recorded, then L.sub.1 =3L.sub.2, in accordance with the aforementioned radial dependency of pit length. Also, the linear velocity of the track whose radius is 15 cm. may be represented as v.sub.1 and the linear velocity of the track whose radius is 5 cm. may be represented as v.sub.2. Again, because the larger radius is three times as great as the smaller radius, v.sub.1 =3v.sub.2. Now, if a pulse is produced when the effective length of a pit is detected, then a pulse of duration t.sub.1 is produced when the pit which is recorded in the track whose radius is 15 cm. may be expressed as ##EQU1## and the duration t.sub.2 of the pulse which is produced when the pit recorded in the track whose radius is 5 cm. is detected may be expressed as ##EQU2## The expression for t.sub.1 may be rewritten as ##EQU3## Since both pits have been assumed as being representative of the same frequency component, it is expected that t.sub.1 =t.sub.2. However, as is clear from the foregoing, t.sub.1 &lt;t.sub.2. This discrepancy is interpreted as a recording of different frequency components in the respective tracks. Thus, it is seen that because of the radial dependency of pit length, erroneous output signals can be reproduced from the record disc.
In the foregoing description, it was assumed that the size of the beam spot is maintained constant for all tracks notwithstanding the change in the radius from one track to the next. If the beam spot size changes in direct proportion with a change in the radius from one track to the next, it is possible that the errors described above may be minimized. However, complex and expensive apparatus is needed to control the size of the scanning beam spot as a function of the radius of the track being scanned. Because of this, it is preferred to maintain a constant beam spot size irrespective of the track radius. Consequently, a compromise generally is made between the length of a pit and the size of the beam spot. This compromise is complicated by the fact that not only does the length of the pit vary with the radius of the track, but the length of the pit also varies as a function of the frequency component of the frequency modulated video signal. Thus, although an optimum relationship between the pit length and the beam spot size can be established for a particular frequency component, this relationship might be less than satisfactory for a different frequency component which is represented by a pit of another length recorded in the very same track.
The changing relation between the length of a pit and the size of the scanning beam spot may result in still further difficulties. For a proper relation, that is, where the size of the scanning beam spot is neither too large nor too small for the pits being scanned, a pit will be detected by a change in the intensity of the light reflected therefrom, such as a reduction in the intensity of the reflected light when the beam spot impinges upon the pit. A typical output signal which is produced when the pit is detected is a negative-going pulse. However, if the length of the pit is too long relative to the size of the scanning beam spot, then the intensity of the light reflected from the surface of the record disc is reduced at the leading edge of the pit but then returns to its "normal" level as the beam scans the bottom of the pit. This is because the bottom of the pit is reflective substantially to the same degree as the surface of the disc. However, when the beam spot impinges upon the trailing edge of the pit, another reduction in the intensity of the reflected light is produced. Consequently, the output signal produced by the scanning of this pit whose length is too long relative to the size of the beam spot is provided with two negative-going pulses. Since a pit is assumed to be represented by a signal negative-going pulse, this elongated pit may be erroneously interpreted as two successive pits. Thus, an erroneous frequency component will be reproduced, thereby resulting in the reproduction of a degraded video picture.
In the foregoing assumption, the relationship between the length of a pit and the size of the scanning beam spot has been selected to be correct for pits of relatively small length, that is, for those pits which are recorded in tracks having a smaller radius. If the relationship is selected to be proper for those pits which are recorded in tracks having a larger radius, then those pits which are of relatively smaller length, that is, those pits which are recorded in tracks having a smaller radius, might not be detected. This is because the size of the scanning beam spot is too large when compared to the lengths of the pits which are recorded on those tracks having smaller radii. When such small pits are scanned, a substantial portion of the beam spot impinges upon the surrounding land, or surface of the disc. As a result, the intensity of the light reflected therefrom is changed only insignificantly by the scanned pit. To best appreciate this problem, let it be assumed that the intensity of the light which is reflected from the disc when the scanning beam impinges upon the land or surface thereof is represented as M, and the intensity of the light which is reflected when the scanning beam impinges upon the pit is represented as P, then the modulation factor m may be defined as m=M-P/M+P. If, as just mentioned, the size of the scanning beam spot is too large relative to the length of the pit being scanned, then the reflected light intensity P is almost equal to M. This means that the modulation factor m is very small. Hence, the signal-to-noise ratio (S/N) for a large beam spot compared to the length of a pit is small, and a deteriorated video picture signal may be reproduced. As a further disadvantage of using a beam spot which is too large relative to the length of a pit, the scanning beam spot may be so large as to impinge upon adjacent pits which are recorded either in the same or adjacent tracks. This introduces crosstalk, interference and further deterioration in the video signal which is reproduced from the disc.